In recent years thin film semi-permeable membranes and separation processes using semi-permeable membranes, such as reverse osmosis, have attracted increasing attention in many fields where separation of materials from solvent is important. Reverse osmosis of semi-permeable membranes can be used in systems for the purification of water, to remove impurities from blood, in the concentration of dilute solutions of fine chemicals such as pharmaceuticals and in many other areas where energy conservation or the nature of the solute prohibits evaporative or distillation processes. Separation processes in which purified water is made from aqueous solutions of solutes, such as sea water, are of particular importance.
In performing reverse osmosis processes through thin, semi-permeable membranes, the aqueous solutions are commonly contacted with the semi-permeable membrane under superatmospheric pressure. In sharp contrast to osmotic processes, substantially purified water passes through the membrane, leaving solute molecules or other impurities in the comparatively concentrated aqueous solution retained by the membrane. A basic discussion of such reverse osmosis treatment is set forth in the treatise "Desalinization by Reverse Osmosis," MTIS Press, 1966, edited by Ulrich Merten, which is expressly incorporated by reference herein.
In semi-permeable membrane reverse osmosis processes, two characteristics are most important. First, the membrane must have a substantial through-put or flux of water to operate efficiently. Secondly, the membrane must reject essentially all solute. Many solutes are particularly toxic, harmful, undesirable, or particularly expensive and their transmittal across the membrane in any substantial concentration can result in the separation being of little value. Accordingly, preferred membranes have high flux or through-put of substantially pure water while rejecting substantial quantities of solute. In desalinization processes a membrane that can achieve a flux of about 9 gallons or greater per square foot of the membrane per day (gfd), preferably 15 gfd or greater, and a salt rejection of 90%, preferably about 95% or greater, can be considered a successful membrane.
The earliest useful semi-permeable membranes which were applied commercially include the Loeb-type membranes made of cellulose diacetate by the processes described in U.S. Pat. Nos. 3,133,132 and 3,133,137. Loeb-type membranes comprise an asymmetric membrane characterized by a thin, dense surface layer or skin supported on an integrally attached, thick porous support layer. Other types of reverse osmosis membranes and methods of preparation have been described in, for example, U.S. Pat. No. 3,246,764, disclosing porous glass fibers coated with polyphosphine oxide, cellulose acetate, or polystyrene. U.S. Pat. No. 3,310,488 discloses a cellulose acetate coated asymmetric membrane. U.S. Pat. No. 3,556,992 discloses an inorganic or organic gel layer on a porous support film. U.S. Pat. No. 3,567,632 discloses asymmetric membranes made from aromatic polyamides. Many of the prior art membranes have been shown to have insufficient through-put or flux, to have insufficient salt rejection, to be subject to chemical or biological degradation, or to have other drawbacks.
One class of polymers which has been identified for in-depth investigation in view of the applicability of the class of polymers to the preparation of specialized barrier materials for use in hostile environments, are polyamide polymers. A discussion of the background, fabrication, properties, and use of polyamide membranes is set forth in Chapter 9, pages 167-210 of "Reverse Osmosis and Synthetic Membranes," National Resource Council of Canada (1977) which is expressly incorporated by reference herein. Semi-permeable membranes have been made by forming thin film barriers from di- and trifunctional aromatic and aliphatic carboxylic acid compounds, isocyanates, aldehydes, etc., and amines such as diaminobenzene, piperazine, hydrazine, and many others. One drawback of certain polyamide-type membrane compositions involves the amine used in forming the membrane. Often the primary amines used in forming the polyamide polymer result in the polymer having proton substituted amido groups. This proton tends to be labile and easily removed under oxidizing conditions, resulting in a deterioration of the amide polymer beginning at the amido group, which can ultimately result in the deterioration of the properties of the thin film membrane.
Wrasidlo, U.S. Pat. No. 4,005,012, teaches one specific type of polyamide membrane. The Wrasidlo semi-permeable membranes comprise an ultra-thin film formed on a support, comprising the reaction product of an amine modified polyepihalohydrin polymer and a polyfunctional carboxylic acid compound which is capable of reacting with the amine groups of the amine modified polyepihalohydrin. The structure of the resulting polyamide structure is shown in Wrasidlo at column 3, lines 28-52, wherein it appears that the polyfunctional acid compound crosslinks the amine modified polyepihalohydrin polymer through the pendent amine groups. It is clear from the formulas that the amino compounds are not part of the polymer backbone, that the polymer backbone is a homopolymer of an epihalohydrin, and that proton-substituted amine groups which can be subject to oxidative attack and membrane deterioration can be present. Accordingly, a substantial need exists for amine compounds which can be used in forming asymmetric thin film membranes having high solvent flux, low solute rejection, and resistance to oxidative deterioration in the presence of oxidants such as chlorine.